Increasing evidence indicates that probably most acute and chronic metal poisoning involve free radical reactions in tissue injury. Since the concentration of radicals occurring in body tissues is generally near the sensitivity limit of ESR spectroscopy, the spin trapping technique is used. The manipulation of an animal model also provides evidence that the toxicity is indeed free radical mediated. By successfully applying this approach, ESR evidence has been obtained for free radical formation generated by iron may occur in vivo not only after acute but also after chronic iron intake. Chromium (VI) compounds pose a serious occupational health hazard in terms of both their potential respiratory tract carcinogenicity and their toxic effect on other organs. By applying ESR and a spin trapping technique, the existence of a radical adduct has also been shown in bile of rats after acute Cr(VI) poisoning. Additional measurements are carried out to provide more data about the exact mechanism by which the detected free radicals from Cr(VI) intoxication are formed. We were not able to show free radical formation in bile or in different tissues (brain, liver, kidneys) after poisoning of rats with lead or with aminolevulinic acid. We also found lack of toxic synergistic effects of Pb and Fe or Al and Fe after these acute metal-metal poisonings.